05000390/LER-2024-002, Automatic Reactor Trip Due to Main Generator Protection Relay Actuation

Automatic Reactor Trip Due to Main Generator Protection Relay Actuation

ML24249A175
Person / Time
Site:	Watts Bar
Issue date:	09/05/2024
From:	Reneau W Tennessee Valley Authority
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
WBL-24-041 LER 2024-002-00
Download: ML24249A175 (1)

LER-2024-002, Automatic Reactor Trip Due to Main Generator Protection Relay Actuation

Event date:
Report date:
3902024002R00 - NRC Website

text

J\\14 TENNESSEE VALLEY AUTHORITY Tennessee Valley Authority, Post Office Box 2000, Spring City, Tennessee 37381 WBL-24-041 September 05, 2024 ATTN : Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Watts Bar Nuclear Plant, Unit 1 Facility Operating License No. NPF-90 NRC Docket No. 50-390 10 CFR 50.73 Subject:

Licensee Event Report 390/2024-002-00, Automatic Reactor Trip Due to Main Generator Protection Relay Actuation Pursuant to the reporting requirements of 10 CFR 50. 73, attached is the subject Licensee Event Report concerning the Automatic Reactor Trip Due to Main Generator Protection Relay Actuation on July 08, 2024.

There are no new regulatory commitments contained in this letter. Please direct any questions concerning this matter to Jonathan Johnson, WBN Licensing Manager, at jtjohnsonO@tva.gov.

Respectfully, neau Site Vice President Watts Bar Nuclear Plant

U.S. Nuclear Regulatory Commission WBL-24-041 Page 2 September 05, 2024 Enclosure: Tennessee Valley Authority. Watts Bar Nuclear Plant, Unit 1, LER 390/2024-002-00, Automatic Reactor Trip Due to Main Generator Protection Relay Actuation cc (w/Enclosure):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Watts Bar Nuclear Plant NRC Project Manager - Region II

ENCLOSURE Tennessee Valley Authority Watts Bar Nuclear Plant Unit 1 LER 390/2024-002-00, "Automatic Reactor Trip Due to Main Generator Protection Relay Actuation" WBL-24-041 E1 of 1

Abstract

While operating at 100% steady state power, the Watts Bar Unit 1 reactor experienced an automatic reactor trip at approximately 1521 EDT on July 8, 2024, in response to a turbine trip. The turbine trip was caused by an electrical issue associated with main generator differential protective relay.

All control and shutdown bank rods inserted properly in response to the reactor trip. All safety systems including the auxiliary feedwater (AFW) system actuated in response to the trip, as expected. All systems actuated as required and there were no complications with the trip. The plant was stabilized with decay heat removed initially through AFW and then through standby main feedwater (MFW) and the steam dump system.

Notification to the NRC of the reactor trip was made by operations at 1824 EDT (Event Notification#57214). The actuation of the reactor protection system (RPS) was reported as a four hour report under 10 CFR 50.72 (b)(2)(iv)(B). The actuation of the AFW system (an engineered safety feature (ESF)) was reported as an eight hour report under 10 CFR 50. 72 (b )(3)(iv)(A).

I.

Plant Operating Conditions before the Event Watts Bar Unit 1 was at 100 percent Rated Thermal Power (RTP).

II.

Description of Event A. Event Summary While operating at 100% steady state power, the Watts Bar Unit 1 reactor experienced an automatic reactor trip at approximately 1521 EDT on July 8, 2024, in response to a turbine trip. The turbine trip was caused by an electrical issue associated with main generator differential protective relay[EIIS: 87].

All control and shutdown bank rods inserted properly in response to the reactor trip. All safety systems including the AFW[EIIS:BA] system actuated in response to the trip, as expected. All systems actuated as required and there were no complications with the trip. The plant was stabilized with decay heat removed initially through AFW and then through standby MFW[EIIS: SJ] and the steam dump system[EIIS: SB].

Notification to the NRC of the reactor trip was made by operations at 1824 EDT (Event Notification#57214). The actuation of the RPS[EIIS: JC] was reported as a four hour report under 10 CFR 50.72 (b)(2)(iv)(B). The actuation of the AFW system (an ESF) was reported as an eight hour report under 10 CFR 50.72 (b)(3)(iv)(A).

B. Status of structures, components, or systems that were inoperable at the start of the event and that contributed to the event There were no safety related inoperable structures, components, or systems that contributed to this event.

00 YEAR 2024 -

3. LER NUMBER SEQUENTIAL NUMBER 002 C. Dates and a.. '.Proximate times of occurrences Dates and Approximate Times 07/08/2024 1521 EDT 07/08/2024 1546 EDT 07/08/2024 1559 EDT 07/08/2024 1824 EDT Occurrence Both the Unit 1 turbine and reactor trip. The operating crew enters the emergency procedure network.

The operating crew exits the emergency procedure network.

A main generator differential relay is found in a tripped state.

The required non-emergency NRC notifications are made.

D. Manufacturer and model number of each component that failed during the event Westinghouse Type "BY" bushing type current transformer (CT) - Style 5614D24G02 E. Other systems or secondary functions affected None.

F. Method of discovery of each component or system failure or procedural error Emergent issue complex troubleshooting identified a faulty or degrading CT[EIIS: XCT].

REV NO.

00 G. The failure mode, mechanism, and effect of each failed component Test results for the particular CT showed early saturation during the excitation test before reaching the knee point. Also, the saturation graph was nonlinear before the knee point.

These results demonstrated that the CT was degraded and responsible for providing the input which resulted in the main generator differential trip.

H. Operator actions Operations personnel promptly stabilized the plant following the reactor trip.

I.

Automatically and manually initiated safety system responses The automatic turbine trip resulted in an automatic reactor trip. Safety systems responded as expected.

Ill.

Cause of the event A. Cause of each component or system failure or personnel error Test results for the particular CT showed early saturation during the excitation test before reaching the knee point. Also, the saturation graph was nonlinear before the knee point.

These results demonstrated that the CT was degraded.

B. Cause(s) and circumstances for each human performance related root cause No human performance issues were identified during this event.

IV.

Analysis of the event This event was compared with previous WBN plant trips and was also compared with applicable Final Safety Analysis Report (UFSAR) transients/accidents. The sequence of events associated with the trip were bounded by the FSAR Safety Analysis assumptions. The parameter response for the reactor trip is bounded by the FSAR analyses in UFSAR Section 15.2.7, "Loss of External Electrical Load and/or Turbine Trip." The plant response post-trip was uncomplicated and the plant responded as designed.

00 V.

Assessment of Safety Consequences A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event Not applicable.

B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident Not applicable.

C. For failure that rendered a train of a safety system inoperable, estimate of the elapsed time from discovery of the failure until the train was returned to service Not applicable.

VI.

Corrective Actions This event was entered into the TVA Corrective Action Program and is being tracked under Condition Report 1942549.

A. Immediate Corrective Actions After Unit 1 had been stabilized in Mode 3, the development of a complex troubleshooting plan was initiated.

B. Corrective Actions to Prevent Recurrence or to reduce the probability of similar events occurring in the future Four CTs which are inputs to the main generator protection scheme were replaced.

A temporary modification (TMOD) was implemented to replace another input to the main generator protection scheme. This TMOD removed from service and grounded one CT and used in its place the output from another existing CT.

REV NO.

00 VII. Previous Similar Events at the Same Site

2. DOCKET NUMBER YEAR 00390 2024

3. LER NUMBER SEQUENTIAL NUMBER 002 REV NO.

00 LER 391/2018-003-00 documents a unit trip that occurred on June 22, 2018 due to actuation of the main transformer differential current relay.

LER 391/2023-002-00 documents a unit trip that occurred on June 27, 2023 most probably due to a loose connection in the circuit for a main transformer differential current relay trip.

LER 391/2024-001-00 documents a unit trip that occurred on January 27, 2024 most probably due to failed wire insulation for a portion of the circuit for a main transformer differential current relay.

VIII. Additional Information None.

IX.

Commitments None. Page _6_ of _6_